Brief Description of the Prior Art
It is known that many types of aquatic animals, such as whales, porpoises and frogs produce sounds for social communication involving the functions of reproduction, mate selection, territoriality and defense, among others. Fishermen have used this fact to attract and catch fish by using fishing lures which click or pop or rattle.
Sound is often of significant and particular value under aquatic environmental conditions unsuitable for visual or olfactory chemical communication. For example, the effectiveness of visual stimuli in an underwater location is frequently limited by low light levels, or heavy suspensions of colloidal material or mud particles or the like which severely attenuate the intensity of the light signal over distance. Chemical signals involving the olfactory sense of the fish propagate slowly, are non-directional and are easily diffused by water currents. Even so, in some instances, such as the use of putrefied liver and blood baits for the catching of catfish, the olfactory sense has been useful in the attraction of fish.
When contrasted with the senses of sight and smell, sound, when used in water, has a high speed of propagation, a low rate of attenuation and has directional properties which permit it to be transmitted from a given location in the water outwardly in a given or selected direction. Consequently, sound is valuable for rapid, high speed communication over a considerable distance. There is general familiarity, of course, With the use of acoustic signals in submarine warfare situations, where hydrophones carried by one vessel are used to pick up the sound generated by the screws of another vessel, as such sounds are transmitted through the water to the receiving ship.
Sound is perhaps the most important source of information to fish about the environment around them, because in contrast to the relatively poor properties of water as a transmitter of light, sound is transmitted through water very efficiently. In comparison with air, water is much denser and much less elastic. It thus transmits sound at a velocity which is about 4.8 times its velocity through air, and the attenuation of sound over distance in water is much less than the attenuation of light. As indicated earlier, it has been recognized for a number of years, that depending upon the acoustic properties of the signals, including the frequency and transient characteristics, certain types of sound such as pops, clicks and rattles can be used to effectively attract fish to the location of the sound source.
Sound detection in fishes involves both the inner ear and the acoustico lateralis systems. The inner ear consist of three, semi-circular canals and three otolithic organs, the utriculus, the sacculus and the lagena. The utriculus is essentially a vestibular organ, along with the semi-circular canals, while the sacculus and the lagena are involved with audition. The lateral lines along the sides of the body of the fish include neuromasts which are composed of air cells, sensory cells and cupula, and such lateral lines are sound sensitive.